1. Field of the Invention
This invention relates to an improved central shaft for collapsible umbrella and particularly to a central shaft that has a novel form of telescopic tubes for increasing tube number and enhancing tube engagement strength.
2. Background of the Invention
Collapsible umbrella is a widely used personal outfit. Conventional collapsible umbrella usually has a central shaft consisting of a plurality of telescopic tubes. Each tube has an engagement means to prevent the tubes from separating from each other during the central shaft extension. U.S. Pat. No. 5,702,198 discloses a central shaft structure (shown in FIG. 1) in which an inner tube A has a concave side of different curvatures G and H, and an outer tube B also has a concave side of different curvatures G and H so that the inner tube A may be slided inside the outer tube B without swiveling or disengaging. In practice, the difference of curvatures G and H is not very huge (FIG. 3). The central shaft usually consists of four tubes or less, such as an inner tube C, a first middle tube D, a second middle tube E and an outer tube F (FIG. 2). The first and second middle tubes D and E have respectively at least two concave sides of different height for engagement. The engagement strength is not very strong. Pulling the tubes with a strong force can easily separating the tubes and make the central shaft not functional. There is still room for improvement.
It is an object of this invention to provide an improved central shaft for collapsible umbrella that has telescopic tubes formed with novel shapes of selected length or height such as a circular perimeter section mating against a flat indent plane, or a flat indent plane mating against an inside concave section so that tube to tube engagement force may be greatly enhanced and tube number may be increased up to five.
Total size of the umbrella may be further reduced to make the umbrella more compact and convenient and useful.